The operating performance of a PN junction adjacent the surface of a semiconductor substrate is frequently degraded by the generation of unwanted charge carriers, thereby increasing the leakage current and lowering the breakdown voltage. From the early history of germanium and silicon single-crystal semiconductor physics, it is known that such semiconductor material contains dangling bonds which are potent minority-carrier lifetime killers. These dangling bonds are responsible for states in the energy gap which, for a reverse-biased PN junction, generate unwanted charge carriers. Dangling bonds occur primarily at the surface of the device and are responsible for the so-called "surface recombination". Dangling bonds also occur at vacancies, micropores, disclocations, and are associated with certain impurities.
We have recently observed that the formation of amorphous silicon (a--Si) by the glow-discharge decomposition of silane results in a hydrogen-rich material. A method of depositing a body of amorphous silicon on a substrate in a glow-discharge apparatus is described in U.S. Pat. No. 4,064,521, which issued to D. E. Carlson on Dec. 20, 1977 and is assigned to RCA Corporation. We have determined that a--Si produced by the glow-discharge decomposition of silane may contain between about 5 to about 50 atomic percent of hydrogen, as reported by J. I. Pankove and D. E. Carlson, "Photoluminescence of Hydrogenated Amorphous Silicon", Applied Physics Letters, Vol. 31, No. 7, 1 Oct. 1977, p. 450-451. Most of this hydrogen is believed to be valence-bonded to silicon atoms in the amorphous network as Si--H bonds. Heating the hydrogenated a--Si in vacuum causes a measureable evolution of hydrogen. When a--Si:H is thermally dehydrogenated, the residue is in the form of Si which is extremely rich in dangling bonds. With this in mind, we have correlated the decreasing photoluminescence efficiency of a--Si:H with hydrogen evolution during annealing, suggesting that the dangling bonds left in the material by the outgassing of hydrogen are effective sites for non-radiative recombination, as they are in single-crystal Si.
The present invention provides a technique for neutralizing the dangling bonds at a PN junction adjacent the surface of a semiconductor substrate, thereby passivating the PN junction by reducing the number of recombination/generation centers adjacent the PN junction. This technique reduces significantly the leakage current across the PN junction when reverse-biased, "sharpens" considerably the breakdown-voltage curve, and improves the operational life expectancy of the PN junction.